As communications networks become more complex, the task of network management becomes increasingly difficult. An important aspect of network management is an identification and accurate record of optical channels in the network, and knowledge of the allocation of channels to the network elements, e.g., to optical links. This may involve knowledge of the allocation of channels to respective optical fibers and optical band filters within the nodes.
One of the common methods for the identification of a channel in an optical network is to modulate the channel with a low frequency tone (dither tone), where the tone uniquely identifies the channel wavelength in the network. A network management system (NMS) associated with network nodes is responsible for mapping each channel with a unique tone, and for keeping track of tones available for channel identification.
In this method, each channel is individually modulated with the dither tone at or near its source, but detection of channels within a wavelength division multiplexed (WDM) optical signal can be performed by demodulating the aggregate signal (without demultiplexing into individual wavelengths) and detecting the spectrum of tones, for example using a Fast Fourier Transform (FFT). This makes it economical to determine the presence of channels in WDM signals at various points in the network.
Channel identification serves primarily two purposes: the confirmation of the presence of a channel at points in the network where the channel is expected to be present; and confirmation of the absence of a channel where the channel is not expected to be present. Faults in the optical switching and transmission equipment that make up the network may have the effects of disrupting the transmission of a channel, or switching it incorrectly. The system of channel identification deployed throughout the network thus enables the NMS to detect and locate such faults.
A method suitable for identification of channels in larger optical networks is described in the pending U.S. patent application Ser. No. 10/259,290 to Obeda et al., entitled “METHOD AND SYSTEM FOR IDENTIFICATION OF CHANNELS IN AN OPTICAL NETWORK” filed Sep. 10, 2002.
Briefly recapitulated, the aforementioned method provides several embodiments for identifying optical channels in large optical networks by means of a channel signature. The channel signature of an optical channel consists of two or more low frequency tones (dither tones) modulated upon the optical channel.
Methods for modulating and reliably detecting dither tones are described in another U.S. pending application Ser. No. 09/972,991 to Wan et al., entitled “CHANNEL IDENTIFICATION IN COMMUNICATIONS NETWORKS” filed Oct. 10, 2001.
In optical networks, optical amplifiers provide for the amplification of the WDM (Wavelength Division Multiplex) optical signals which include the dither tones. While the optical amplifiers are capable of amplifying the multi-wavelength optical signal as a whole, one side effect of this amplification is cross talk of the low-frequency dither tones between the wavelength channels.
As a result of this cross talk, the identification of channels down stream of such amplification points in the network is rendered more difficult, because the tones constituting a channel signature belonging to a particular wavelength channel may also appear (in attenuated form) on other channels.
The effect of cross talk among the modulated WDM channels in an optical amplifier is termed “cross gain modulation” (XGM), and is described in detail in an article entitled “A Transport Network Layer Based on Optical Network Elements” by G. R. Hill et al in Journal of Lightwave Technology, 1993, pp. 667–676.
The effect of XGM on the channel identification method based on the decoding of dither tones is briefly demonstrated with the help of FIG. 1 illustrating a number of spectrum views of tones one might expect to observe in an optical network 10. The diagram of spectrum views (tone spectra) of FIG. 1 is organized as four rows and three columns. In the first three rows are shown tone spectra of individual wavelength channels λ1 (spectra 12, 14, and 16), λ2 (spectra 18, 20, and 22), and λ3 (spectra 24, 26, and 28), the fourth row shows the aggregate tone spectra that would be observed on the WDM (multiplexed) optical channel as a whole, i.e. λ1+λ2+λ3 (spectra 30, 32, and 34).
Spectral lines in each of the tone spectra appear at one or more frequency points, labeled in spectrum 30 as f1, f2, and f3, and applying analogously in all other spectra of FIG. 1. In this simple example, a tone of frequency f1 is used for identifying λ1, f2 identifies λ2, and f3 identifies λ3.
Inserted between the aggregate tone spectra (30, 32, and 34) are symbols indicating stages of the network 10 processing the optical signal, i.e. an optical amplifier 36, and a drop-channel module 38.
In the first column of spectra each of the individual channel spectra (spectra 12, 18, and 24) contains a single spectral line representing the tone assigned to that channel, a line at frequency f1 corresponding to λ1, and so on. The aggregate spectrum 30 contains the sum of the individual spectra 12, 18, and 24, that is spectral lines at f1, f2, and f3.
The spectra of the second column (14, 20, 26, and 32) represent spectra after the optical signal has passed through the optical amplifier 36.
Each of the individual channel spectra (spectra 14, 20, and 26) contains the original spectral line representing the tone assigned to that channel, as well as spectral lines representing the tones from all other wavelength channels that pass through the amplifier 36 as part of the WDM signal. These other (unwanted) spectral lines, also termed “ghosts” or “ghost tones”, are caused by the XGM effect mentioned above. The aggregate spectrum 32 contains the sum of the individual spectra 14, 20, and 26, indicating the presence of wavelength channels λ1, λ2, and λ3. Note that at this point the effect of XGM is not discernible in the aggregate spectrum 32 (apart from second order effects with which we are not currently concerned).
The spectra of the third column (16, 22, 28, and 34) represent spectra after the optical signal passes through the channel-drop module 38. We assume here that the channel-drop module 38 has ideal behavior and removes the wavelength channel λ3 completely.
The spectra 16 and 22 after the channel-drop module 38 are unchanged from the spectra 14 and 20 respectively before the channel-drop module 38. The spectrum 28 contains no spectral line since the signal at the wavelength λ3 has been dropped. The aggregate spectrum 34 contains the sum of the individual spectra 16, 22, and 28. Note the presence of two strong spectral lines at frequency points f1 and f2 in the aggregate spectrum 34, indicating the presence of wavelength channels λ1 and λ2. A third, shorter spectral line 35 at frequency point f3 is the result of ghosts representing wavelength channel λ3, contributed by the ghost tones 11 and 13 of both the wavelength channels λ1 and λ2. In other words, even though the wavelength channel λ3 is not present in the optical signal, the ghost at f3 , present in the aggregate spectrum 34 due to the effect of XGM, could be mistaken as an indicator of the presence of the wavelength channel λ3.
Thus, the presence of ghost tones due to XGM may compromise the accuracy of channel identification in optical networks, especially larger optical networks with many optical amplifiers, and networks with a large number of wavelength channels and a large number of tones, including networks using multiple tones as channel signatures. This effect is cumulative, i.e. the amplitude of ghost tones increases with each amplifier stage passed by the signal.
Accordingly, there is a need in industry for the development of a method and system to compensate for side effects of XGM in amplified optical networks, which would enable accurate channel identification in optical networks.